At the risk of not answering the question directly.... How can you be certain that current physics theory is sufficient to describe everything that makes a difference to sound? - since a true scientist should always be sceptical regarding the sufficiency of existing theory. Therefore, if no one can provide any physics theory to explain the influence of these products, how could you conclude they do not make a difference? On the other hand, even if there is some available physics theory that explains that these devices have an impact on the sound, how can you conclude that therefore they will have a noticeable, or even beneficial, impact on the sound you hear or your enjoyment of the music? Surely, you need to focus on the credibility of the individuals making the claims with regard to these items, carefully consider the context in which the recommendation is made and compare that with your own situation (eg. I doubt the economics of using a Shakti Stone on a boombox), and then make a judgement as to whether you should invest the time to try it yourself. Then when you try it, it is up to your judgement. What I find interesting is the very common lack of trust many non-audiophiles have in whether or not they could make a sound judgement based on listening for themselves. Since the whole purpose of our pursuit is musical enjoyment (by using our ears, not a volt meter), then it seems to me that whether or not our musical enjoyment is enhanced has to be the deciding factor for us individually. All the rest, such as the theory and the reviews can only be pointers to help us along the way.
"Feng Shui" and "Floobie Dust"
These titles were once used in an edition of Stereophile reporting on some miscellaneous items from a CES show. Do things like Shun Mook disks, Shakti stones, silver vs. copper wire (the resistivity of silver is only a little greater, so is it the grain structure?), power conditioners (for example, there was a fairly interesting article in the current issue of The Absolute Sound debunking most power conditioners and filters), etc., make a difference? Is there any physical basis (in terms of an understanding of the physics) for the claims made by the manufacturers?